An attempt at an intuitive interpretation of the Feynman diagrams was made by Bergmann.214 It is argued that one important class of diagrams may be interpreted as diffraction of one electron by the oscillations in the electrostatic potential generated by the other electrons. The Coulomb interaction between the electrons thus in33 troduces a purely quantum mechanical correlation between their motion, which is observable in the conductivity. The diffraction of one electron wave by the interference pattern generated by another electron wave will only be of importance if their wavelength difference, and thus their energy difference, is small. At a finite temperature T , the characteristic energy difference is kBT . The time τT ≡ ¯h/kBT enters as a long-time cutoff in the theory of electron-electron interactions in a disordered conductor, in the usual case127,211 τT