Can the conductance step of a single-mode ballistic constriction be lower than 2e2/h?

We have performed four-terminal measurements on a split-gate device with three overlaying finger gates. In our system, a one-dimensional (1D) ballistic constriction can be formed by applying a negative voltage on the split-gate. By changing the finger gate voltage, we are able to modulate the potential profile within the 1D constriction. In such a 1D ballistic structure we have observed that the conductance steps show a gradual decrease from 2e2/h to 0.97×2e2/h with increasing negative finger gate voltage. We suggest this phenomenon is due to different shifts of 1D subbands with changing spilt-gate voltage. Both simple analytical estimate for an adiabatic constriction and realistic modeling of the device give the same magnitude of the conductance decrease as observed in our experiments.