  We establish a connection between the influence functional approach of
Golubev and Zaikin (GZ) and Keldysh diagrammatic perturbation theory for
calculating the decoherence time of interacting electrons in disordered metals;
we show how the standard diagrams for the Cooperon self energy can be recovered
from GZ's influence functional $e^{- (i S_R + S_I)}$. This allows us to shed
light on GZ's claim that $S_R$ is irrelevant for decoherence: $S_R$ generates
as many important self energy diagrams as $S_I$; GZ's neglect of $S_R$ is
permissible only at high temperatures ($T > \hbar / \tau_{el}$).
