Conductance of quantum interference transistors in parallel and in series

We theoretically study the electronic conductance G and the current-voltage characteristics of two quantum interference transistors in parallel and in series. We use two different definitions of conductance, G similar to T and G similar to T/R, Neither can reproduce the classical additivity law in the case of coherent transport due to quantum interference for the elements in series and quasibound states when elements are in parallel. In the case of two transistors in series, we find that the quantity T/R only qualitatively better represents the additivity law, which is probably expected because this model avoids counting the contact resistance twice. However, for the parallel configuration of transistors, the conductance is almost additive for the majority of energies when G similar to T, except for the single-mode regime. Possible use of these configurations in digital electronics for basic logic functions is discussed. (C) 1999 Academic Press.