Quantum mechanics is not the death of determinism. QM is deterministic, just about different things than classical mechanics, otherwise, it would have no predictive powers. Schrödinger’s equation predicts the probability of certain outcomes. At any rate, most models of the brain are classical in nature. (Penrose is the only one I can think of who proposed that QM might apply on the scale of the structures in the brain, and hence provide a mechanism towards free will.) Besides, if free will is nothing more than the sum of random outcomes to physical processes in the brain, wherein lies the ability to choose?