Two problems here. First, even if you want to invoke quantum indeterminacy as a mechanism for free will, you have to deal with the fact that quantum phenomenon are completely random - at best, this would allow you to establish "random will" that a person doesn't actually have any control over.
Second, determinism certainly is present on the macroscopic scale. You have to get down to very specific, very small scales - like a single atom waiting to decay, or the position of an individual electron around a nucleus - before quantum indeterminacy starts to matter. It's not at all clear that quantum indeterminacy has anything to do with the macroscopic functioning of the brain. Even a single neuron is so large that indeterminate quantum effects would no longer have any significant impact.
This is what 'free will' advocates don't grasp. Things either happen for a reason, or are random. Combining these two mechanisms in any quantity does not produce 'free' thought. Thoughts are bound by mechanisms.