Gradual decrease of conductance of an adiabatic ballistic constriction below 2 e 2 / h

We have performed four-terminal conductance measurements of a one-dimensional (1D) channel in which it is possible to modulate the potential profile using three overlaying finger gates. In such a 1D ballistic structure we have observed that the conductance steps show a gradual decrease from 2e(2)/h to 0.97x2e(2)/h with increasing negative finger gate voltage in a short, clean 1D constriction. We suggest this phenomenon is due to differing shifts of 1D subbands with changing spilt-gate voltage. Both a simple analytical estimate for an adiabatic constriction and a realistic modeling of the device give the same magnitude of the conductance decrease as observed in our experiments.