  We develope a theory of a fundamental effect of the interaction-induced
decoherence of the electron wave function in a disordered metal. With the aid
of the Keldysh technique and the path integral formalism we derive a formally
exact equation of motion for the electron density matrix in the presence of
interaction. We demonstrate that the effect of interaction of the electron with
other electrons and lattice ions in a disordered metal is equivalent to that of
an effective dissipative environment. Quantum noise of this environment causes
quantum decoherence even at T=0. Our analysis explicitely accounts for the
Pauli principle which plays an important role for inelastic scattering
processes but turns out not to affect quantum decoherence. Our results
seriously challenge the existence of strong localization in low dimensional
disordered metals.
