  The overlap of two wave functions evolving in time with slightly different
Hamiltonians decays exponentially, for perturbation strengths greater than the
level spacing. We present numerical evidence for a dynamical system that the
decay rate is given by the smallest of the Lyapunov exponent of the classical
chaotic dynamics and the level broadening that follows from the golden rule of
quantum mechanics. This limits the range of validity for the
perturbation-strength independent decay rate discovered by Jalabert and
Pastawski [Phys. Rev. Lett. 86, 2490 (2001)].
