IntroductionBoth the quantum Hall effect (QHE) and the quantized conductance of a ballistic point contact are described by the same relation, G = Ne2/h, between the conductance G and the number N of propagating modes at the Fermi level (counting both spin directions separately). The smooth transition from zero-field quantization to QHE that follows from this relation is evident from Fig. 48. The nature of the modes is very different, however, in weak and strong magnetic fields. As we discussed in Section III.A.1, the propagating modes in a strong magnetic field consist of edge states, which interact with one of the sample edges only. Edge states with the same mode index are referred to collectively as an edge channel. Edge channels at opposite edges propagate in opposite directions. In a weak magnetic field, in contrast, the modes consist of magnetoelectric subbands that interact with both edges. In that case there is no spatial separation of modes propagating in opposite directions.