Quantum conductance of point contacts in Si inversion layers

We have performed electron-transport studies on high-mobility Si inversion layers in which a narrow and short constriction is defined electrostatically. When the width of the constriction is varied, steplike structures in the conductance are observed with a spacing of approximately 4${\mathit{e}}^2$/h in zero magnetic field and at a temperature of 1.2 K. In the presence of a magnetic field these features develop into quantum Hall plateaus and the fourfold degeneracy is lifted. We argue that this behavior arises from the depopulation of successive one-dimensional subbands in the constriction.