  We compare quantum decoherence in generic regular and chaotic systems that
interact with a thermal reservoir via a dipole coupling. Using a
time-dependent, self-consistent approximation in the spirit of Hartree, we
derive in the high temperature limit an expression for the off-diagonal
elements of the system density operator that initially corresponds to a
coherent superposition of two adjacent wave packets. We relate the decoherence
rate to the Lyapunov exponent in the Ehrenfest regime. In this regime, the
greater the instability of the system the faster the loss of coherence occurs.
