The non linear transport in a Luttinger liquid, modeling a quantum wire, is considered for a general time dependent driving electric field, in presence of an impurity. It is explicitly shown that only in the dc limit the current is independent on the spatial shape of the electric field. Without the impurity linear response theory is exact. The intrinsic ac conductance of the system results proportional to the derivative of the inverse of the dispersion relation. For non zero range interactions between the electrons, it shows a resonance at a finite frequency which is related to plasmon--like charge wave excitation. The complex conductance is analyzed in terms of parameters that are quantum analogs of classical impedances.