Here N is the number of propagating modes in the channel (or “lead”) connected to reservoir α (which in general may be different from the number N in lead β), and t ,mn is the transmission probability amplitude from mode n in lead α to mode m in lead β. The leads are modeled by ideal electron waveguides, in the sense discussed before, so that the reservoir α at chemical potential μ above EF injects into lead α a (charge) current (2e/h)Nμ. A fraction T→ /N of that current is transmitted to reservoir β, and a fraction T→a/N ≡ R/N is reflected back into reservoir α, before reaching one of the other reservoirs. The net current I in lead α is thus given by 5