Consider a source and a drain connected by a wire. When a voltage is applied the chemical potential of source and drain are shifted relative to each such that: μ_source = μ_drain + eV now, one crucial step in deriving the Landauer formula for a system like this is to, as indicated in the link http://www.gianlucafiori.org/qpc/node7.html, to realize that the fermi functions for source and drain are different and that the current going to the right and left respectively are given as integrals of the fermi function. Now, why is this exactly? It is not like the source has more electrons than the drain. Is it because they are moving faster?