LOW-FREQUENCY ADMITTANCE OF A QUANTUM POINT-CONTACT

We present a current and charge conserving theory for the low frequenc y admittance of a quantum point contact. We derive expressions for the electrochemical capacitance and the displacement current. The latter is determined by the emittance which equals the capacitance only in th e limit of vanishing transmission. With the opening of channels the ca pacitance and the emittance decrease in a steplike manner in synchroni sm with the conductance steps. For vanishing reflection, the capacitan ce vanishes and the emittance is negative.