  Combining scattering matrix theory with non-linear $\sigma$-model and Keldysh
technique we develop a unified theoretical approach enabling one to
non-perturbatively study the effect of electron-electron interactions on weak
localization and Aharonov-Bohm oscillations in arbitrary arrays of quantum
dots. Our model embraces (i) weakly disordered conductors (ii) strongly
disordered conductors and (iii) metallic quantum dots. In all these cases at $T
\to 0$ the electron decoherence time is found to saturate to a finite value
determined by the universal formula which agrees quantitatively with numerous
experimental results. Our analysis provides overwhelming evidence in favor of
electron-electron interactions as a universal mechanism for zero temperature
electron decoherence in disordered conductors.
