  We use a recent result to show that the rate of loss of coherence of a
quantum system increases with increasing system phase space structure and that
a chaotic quantal system in the semiclassical limit decoheres exponentially
with rate $2 \lambda_2$, where $\lambda_2$ is a generalized Lyapunov exponent.
As a result, for example, the dephasing time for classically chaotic systems
goes to infinity logarithmically with the temperature, in accord with recent
experimental results.
