This does not affect the conductanc

This does not affect the conductance of the constriction, however, if the breakdown of adiabaticity results in a mixing of the subbands without causing reflection back through the constriction. If such is the case, the total transmission probability through the constriction remains the same as in the hypothetical case of fully adiabatic transport. As pointed out by Yacoby and Imry,326 a relatively small adiabatic increase in width from Wmin to Wmax is sufficient to ensure a drastic suppression of reflections at Wmax. The reason is that the subbands with the largest reflection probability are close to cutoff, that is, they have subband index close to Nmax, the number of subbands occupied at Wmax. Because the transport is adiabatic from Wmin to Wmax, only the Nmin subbands with the smallest n arrive at Wmax, and these subbands have a small reflection probability. In the language of waveguide transmission, one has impedance-matched the constriction to the wide 2DEG regions.328 The filtering of subbands by a gradually widening constriction has an interesting effect on the angular distribution of the electrons injected into the wide 2DEG. This horn collimation effect329 is discussed in Section III.D.