Imagine two wide electron gas reservoirs having a slight difference δn in electron density, which are brought into contact by means of a narrow channel, as in Fig. 42a. A diffusion current J will flow in the channel, carried by electrons with energies between the Fermi energies EF and EF + δμ in the low- and high-density regions. For small δn, one can write for the Fermi energy difference (or chemical potential difference) δμ = δn/ρ(EF), with ρ(EF) the density of states at EF in the reservoir (cf. Section I.D.1). The diffusion constant (or “diffusance”)4 ˜D is defined by J = ˜Dδn and is related to the conductance G by