Kinetic conductance and the characteristic carrier traversal times of small conductors

The current response to oscillating external electric fields induced along the probes of a phase coherent conductor is calculated. For non-interacting carriers we find an expression of the equilibrium ac-conductance in terms of the scattering matrix of the conductor. Our derivation invokes the Kramers-Kronig relations for the scattering matrices. The low-frequency departure of the conductance away from the dc-conductance is Drude like but with the role of the scattering time played by weighted averages of the time taken by carriers for traversal, reflection or dwelling in the conductor.