  In diffusive Cu and Au quantum wires at finite transport voltage $U$ the
non-equilibrium distribution function $f(E,U)$ exhibits scaling behavior,
$f(E,U)=f(E/eU)$, indicating anomalous energy relaxation processes in these
wires. We show that in nonequilibrium the Kondo effect, generated either by
magnetic impurities (single-channel Kondo effect) or possibly by non-magnetic,
degenerate two-level systems (two-channel Kondo effect), produces such scaling
behavior as a consequence of a Korringa-like (pseudo)spin relaxation rate
$\propto U$ and of damped powerlaw behavior of the impurity spectral density as
a remnant of the Kondo strong coupling regime at low temperature but high bias.
The theoretical, scaled distribution functions coincide quantitatively with the
experimental results, the impurity concentration being the only adjustable
parameter. This provides strong evidence for the presence of Kondo defects,
either single- or two-channel, in the experimental systems. The relevance of
these results for the problem of dephasing in mesoscopic wires is discussed
briefly.
