I refuse to enter the discussion before creating a solid formal definition of what "free will" is, to some it is the ability to chose your destiny based on your desires in the moment, to others it is a stronger idea of not being tied to the autonomic drives of your physical self that defines the probability cone of what you *may do*.
It is obvious both from the neuroscience and biology proper that it can't be the former, "free will" is about weather or not your substrate bound cone of possibility is in anyway *pre* constrained OUTSIDE of your physical self.
The answer to THAT question is obvious, NO. Because though you are self contained body and brain, your physical unit is embedded in a random soup of possible interactions with OTHER agents (and a more or less random environment bubble as well) where your physical unit has no way of predicting what it will interact with, what resources it will encounter and thus no way of pre- determining what it will chose across any aspect of the emergent cognitive landscape (MIND).
So the answer to why it has been so hard to answer is that there are scopes of observation to define what "free will" is, and the confusion over the different definitions of "free will" that apply scope of observation of "free will" from physical unit TO mind as opposed to from say brain physiological substrate to MIND is where all the confusion comes in. Different people arguing about different conceptions of "free will" but no realizing it.
So wit the definition I described above down, I say there is free will in that scope of observation....but as we bring the cone of observation closer to the mind free will seems to boil down to a finite if large set of "reactions" to a given stimuli that look like there isn't free will but that is only because the randomness of the external physical context is not being accounted for.
In the same way that just 4 base nucleotides can be combined in *infinite* lengths to produce variant proteins and enzymes...so to is this set of possible choice options for cognitive agents infinitely combined to produce unique "choice" interactions...and thus proving that "free will" (secondarily defined as the ability to select from an infinite set of possible choices chains) does exist.
Sam Harris is stuck in the brain, he's not adding in that "physical unit" scope to external environment and so from his perspective all he sees is that subset of "reactions" to stimuli but he misses that those reactions are continuously summable and each unit addition makes a new "choice" possibility, just as each new amino acid added to a protein chain makes it's fold and active site affinity *slightly* different and thus it's bonding activity different.
So, does "free will" exist? If you accept the wider scope view then yes...if you accept the finite subset of "reactions" view, no. It is obvious that we are indeed embedded in a environment of interaction with other individuals that is extremely large if not infinite but constantly changing, in that context then we have practical free will, as no interaction between ourselves and the external environment will ever be the same and our internal reaction to those external stimuli also dynamically varies, so I'll just go ahead and say "YES" with the aforementioned conditions in place.