Here N is the (B-dependent) number of occupied subbands in the point contacts, and Nwide is the number of occupied Landau levels in the 2DEG between the point contacts. The physical origin of the simple addition rule (3.39) is additivity of the four-terminal longitudinal resistance (3.23). From this additivity it follows that for n different point contacts in series, Eq. (3.39) generalizes to