Dephasing of electrons in mesoscopic metal wires

We have extracted the phase coherence time τ_φ of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper, and gold. In samples fabricated using our purest silver and gold sources, τ_φ increases as T^-2/3 when the temperature T is reduced, as predicted by the theory of electron--electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of τ_φ starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasisaturation of τ_φ over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov--Bohm rings fabricated using a very pure copper source and found that the amplitude of the $h/e$ conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed ``saturation'' of τ_φ in weakly disordered metallic thin films can be attributed to spin--flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means.