Transmission through evanescent mod

Transmission through evanescent modes (i.e., subbands above EF) is predominant for the (N + 1)th subband, since it has the largest decay length _N+1 = h[2m(EN+1 −EF)]−1/2. The observation of that plateau requires that the constriction length exceeds this decay length at the population threshold of the N th mode, or L >∼h[2m(EN+1−EN)]−1/2 ≈ (WλF)1/2. The optimum length313 Lopt ≈ 0.4(WλF)1/2 thus separates a short constriction regime, in which transmission via evanescent modes cannot be ignored, from a long constriction regime, in which transmission resonances obscure the plateaux.