Role of the environment in chaotic quantum dynamics.

We study how the interaction with an external incoherent environment induces a crossover from quantum to classical behavior for a particle whose classical motion is chaotic. Posing the problem in the semiclassical regime for weak bath coupling, we find that noise rather than dissipation is primarily responsible for the dephasing that leads to the quantum inhibition of mixing. However, the noise is also shown to inhibit the mixing by enhancing paths that interfere coherently.