A method311,329 to sensitively detect the collimated electron beam injected by a point contact is to sweep the beam past a second opposite point contact by means of a magnetic field. The geometry is shown in Fig. 61 (inset). The current Ii through the injecting point contact is drained to ground at one or two (the difference is not essential) ends of the 2DEG channel separating the point contacts. The opposite point contact, the collector, serves as a voltage probe (with the voltage Vc being measured relative to ground). In the case that both ends of the 2DEG channel are grounded, the collector voltage divided by the injected current is given by 3 with G = (2e2/h)N the two-terminal conductance of the individual point contact (both point contacts are assumed to be identical) and Td the direct transmission probability between the two point contacts calculated in Section III.D.1.