  We show that the Korringa ratio, associated with nuclear magnetic resonance
in metals, is unity if vertex corrections for the dynamic spin susceptibility
are negligible and the hyperfine coupling is momentum independent. In the
absence of vertex corrections we also find a Korringa behaviour for $T_1$, the
nuclear spin relaxation rate, i.e., $1/T_1\propto T$, and a temperature
independent Knight shift. These results are independent of the form and
magnitude of the self-energy (so far as is consistent with neglecting vertex
corrections) and of the dimensionality of the system.
