which is unaffected by the presence of the constriction. This is a consequence of our assumption of ideal voltage probes. One can also measure the two four-terminal diagonal resistances R+D and R−D across the constriction in such a way that the two voltage probes are on opposite edges of the 2DEG, on either side of the constriction (see Fig. 82). Additivity of voltages on contacts tells us that R±D = RH ± RL (for the magnetic field direction of Fig.82); thus,