  We develop a new unified theoretical approach enabling us to
non-perturbatively study the effect of electron-electron interactions on weak
localization 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
determined by the universal formula $\tau_{\varphi 0}\sim g\tau_D/\ln
(E_C/\delta)$, where $g$, $\tau_D$, $E_C$ and $\delta$ are respectively
dimensionless conductance, dwell time, charging energy and level spacing of a
single dot. In the case (i) this formula yields $\tau_{\varphi 0}\propto
D^3/\ln D$ ($D$ is the diffusion coefficient) and matches with our previous
quasiclassical results [D.S. Golubev, A.D. Zaikin, Phys. Rev. Lett. 81 (1998)
1074], while in the cases (ii) and (iii) it illustrates new physics not
explored earlier. A detailed comparison between our theory and numerous
experiments provides an overwhelming evidence that zero temperature electron
decoherence in disordered conductors is universally caused by electron-electron
interactions rather than by magnetic impurities.
