  The Loschmidt echo is a measure of the stability and reversibility of quantum
evolution under perturbations of the Hamiltonian. One of the expected and most
relevant characteristics of this quantity for chaotic systems is an exponential
decay with a perturbation independent decay rate given by the classical
Lyapunov exponent. However, a non-uniform decay -- instead of the Lyapunov
regime -- has been reported in several systems. In this work we find an
analytical semiclassical expression for the averaged fidelity amplitude that
can be related directly to the anomalous -- unexpected-- behaviour of the LE.
