To calculate the diffusance, we fir

To calculate the diffusance, we first consider the caseof an ideal electron waveguide between the two reservoirs.By “ideal” it is meant that within the waveguidethe states with group velocity pointing to the right areoccupied up to EF +δμ, while states with group velocityto the left are occupied up to EF and empty above thatenergy (cf. Fig. 42b). This requires that an electron nearthe Fermi energy that is injected into the waveguide bythe reservoir at EF + δμ propagates into the other reservoirwithout being reflected. (The physical requirementsfor this to happen will be discussed in Section III.B.) Theamount of diffusion current per energy interval carried bythe right-moving states (with k < 0) in a mode n is theproduct of density of states ρ−n and group velocity vn.Using Eqs. (1.5) and (3.3), we find the total current Jncarried by that mode to be