  In recent experiments the non-equilibrium distribution function $f(E,U)$ in
diffusive Cu and Au quantum wires at a transport voltage $U$ shows scaling
behavior, $f(E,U)=f(E/eU)$, indicating a non-Fermi liquid interaction with {\it
non-vanishing} T=0 scattering rate. The two-channel Kondo (2CK) effect,
possibly produced by degenerate two-level systems, is known to exhibit such
behavior. Generalizing the auxiliary boson method to non-equilibrium, we
calculate $f(E,U)$ in the presence of 2CK impurities. We show that the 2CK
equations reproduce the scaling form $f(E/eU)$. For all measured samples the
theoretical, scaled distribution functions coincide quantitatively with the
experimental results, the impurity concentration being the only adjustable
parameter. This provides a microscopic explanation for the experiments and,
considering that no other mechanism producing the scaling form is known to
date, lends strong evidence for the presence of degenerate two-level defects in
these systems. The relevance of these results for the problem of dephasing in
mesoscopic wires is discussed.
