Detection of fractional edge channel by quantum point contacts

Conductance spectrum through a quantum point contact in quantum Hall liquid is investigated by its dependence on the excitation voltage. The occurrence of conductance plateaus different from the bulk Hall conductance indicates a gradual confinement potential at the sample edge. In the low excitation limit, the point contact is pinched off at smaller negative gate voltage for higher magnetic field. This trend is interpreted in terms of the magnetic field induced shift in the energy spectrum.