We develop a realistic, physics based, practical RF circuit model for the AC impedance of a quantum point contact that includes the ohmic contacts, the on-chip "lead" resistance and kinetic inductance, and the quantum point contact impedance itself. The kinetic inductance of the electrons in the "leads" in series with the quantum point contact capacitance form a resonant tank circuit whose resonant frequency depends on the width of the quantum point contact channel. These measurements probe

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... ces in the following qualitative regime: They are in the ballistic limit, and the measurement frequency is higher than the electron scattering frequency.