Figure 49 shows N versus B−1 for va

Figure 49 shows N versus B−1 for various gate voltages, as it follows a square-well confining potential, with the potential barrier in the constriction taken into account by replacing EF by EF –Ec in Eq. (2.62). The B-dependence of EF has been ignored in the calculation. The barrier height Ec is obtained from the high-field conductance plateaux [where N ≈ (EF − Ec)/¯hωc], and the constriction width W then follows from the zero-field conductance (where N ≈ [2m(EF − Ec)/h2]1/2W/π). The good agreement found over the entire field range confirms the expectation that the quantized conductance is exclusively determined by the number of occupied subbands, irrespective of their electric or magnetic origin. The analysis in Fig. 49 is for a square-well confining potential.334 For the narrowest constrictions a parabolic potential should be more appropriate,61 which has been used to analyze the data of Fig. 48 in Refs.336 and308. Wharam et al.337 have analyzed their depopulation data using the intermediate model of a parabolic potential with a flattened bottom (cf. also Ref.336). Because of the uncertainties in the actual shape of the potential, the parameter values tabulated in Fig. 49 should be considered as rough estimates only.