The effect of a magnetic field (per

The effect of a magnetic field (perpendicular to the 2DEG) on the quantized conductance of a point contact is shown in Fig. 48, as measured by van Wees et al.334 First of all, Fig. 48 demonstrates that the conductance quantization is conserved in the presence of a magnetic field and shows a smooth transition from zero-field quantization to quantum Hall effect. The most noticeable effect of the magnetic field is to reduce the number of plateaux in a given gate voltage interval. This provides a demonstration of depopulation of magnetoelectric subbands, which is more direct than that provided by the experiments discussed in Section II.F. In addition, one observes that the flatness of the plateaux improves in the presence of the field. This is due to the reduction of the reflection probability at the point contact, which is revealed most clearly in a somewhat different (four-terminal) measurement configuration. These two effects of a magnetic field will be discussed separately. We will return to the magnetic suppression of back-scattering in Section IV.A in connection with the edge channel theory112 of the quantum Hall effect.