  We calculate the dephasing rate due to magnetic impurities in a weakly
disordered metal as measured in a weak localization experiment. If the density
n_S of magnetic impurities is sufficiently low, the dephasing rate 1/tau_phi is
a universal function, 1/tau_phi = (n_S/nu) f(T/T_K), where T_K is the Kondo
temperature and nu the density of states. We show that inelastic vertex
corrections with a typical energy transfer Delta E are suppressed by powers of
1/(tau_phi Delta E) proportional to n_S. Therefore the dephasing rate can be
calculated from the inelastic cross section proportional to pi nu Im T- |pi nu
T|^2, where T is the T-matrix which is evaluated numerically exactly using the
numerical renormalization group.
