  We address the question of the microscopic origin of dissipation in
collective motion of a quantum many--body system in the framework of a
parametric random matrix approach to the intrinsic dynamics. We show that the
fluctuation--dissipation theorem is generally violated and, moreover, energy
diffusion has a markedly non--Gaussian character and the corresponding
distribution has very long tails. Such features do not support a Langevin or
Fokker--Planck approach to dissipation in collective nuclear motion.
