A description of (quantum or classical) transport involving the probabilities of transfer of any given number of charge carriers. See the following works for a review of this approach (in the context of quantum transport):
-  Ya. Blanter, M. Buettiker, "Shot noise in mesoscopic conductors", Phys. Rep. 336, 1 (2000).
-  L. S. Levitov, H.-W. Lee, and G. B. Lesovik, "Electron counting statistics and coherent states of electric current", J. Math. Phys. 37, 4845 (1996).