We propose an intrinsic mechanism for zero-temperature dephasing of conduction electrons in metals. We consider a polycrystalline impure metal and calculate coherent intergrain charge transfer at zero temperature. It is shown that, in the process of transfer, the wave-function amplitude of a conduction electron decays, due to the interference between two alternative ways of charge transfer, each being the exchange process of the other. This decay of amplitude is used to derive an effective dephasing time tau(phi) for the conduction electron. We obtain 1/tau(phi)similar to(E-c/epsilon(f))(l(2)/d(2))/tau, with E-c being the charging energy of a grain, l is the mean free path, d is the grain size, and tau is the elastic scattering time. Using typical values, we estimate tau(phi)similar too(10(-11) s), in agreement with recent experiments. Our result indicates that zero-temperature dephasing can arise from an intrinsic mechanism, e.g., intragrain electron-electron interaction, as demonstrated in this calculation.