Incompleteness of the Landauer formula for electronic transport

We show that the Landauer formula for the conductance of a nanoscale system is incomplete because it does not take into account many-body effects which cannot be treated as contributions to the single-particle transmission probabilities. We show that the physical origin of these effects is related to the viscous nature of the electron liquid and we develop a perturbative formalism, based on the time-dependent current-density-functional theory, for calculating the corrections to the resistance in terms of the ``Kohn-Sham current distribution'' and the exchange-correlation kernel. The difficulties that still remain in calculating the latter are critically discussed.