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Series expansion of the quantum admittance in mesoscopic systems

The quantum admittance of an interacting/coupled mesoscopic system and its series expansion are obtained by using the refermionization method. With the help of these non-perturbative results, it is possible to study the dependencies of the admittance according to the applied dc voltage, temperature, and frequency without any restriction on the relative values of these variables. Explicit expressions of the admittance are derived both in the limits of weak and strong interactions/coupling strength, giving clear indication of the inductive or capacitive nature of the mesoscopic system. They help to determine the conditions under which the phase of the current with respect to the ac voltage is positive.