2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/89901We discuss the behavior of fidelity for a classically chaotic quantum system in the metallic regime. We show the existence of a critical value of the perturbation below which the exponential decay of fidelity is determined by the width of the Breit-Wigner distribution and above which the quantum decay follows the classical one which is ruled by the Lyapunov exponent. The independence of the decay {\it rate} from the perturbation strength derives from the similarity of the quantum and classical relaxation process inside the Heisenberg time scale.revtex, 5 pages, 5 figuresQuantum PhysicsCondensed MatterChaotic Dynamicstext