  We present a discussion of the admittance (ac-conductance) and nonlinear
I-V-characteristic for a number of mesoscopic conductors. Our approach is based
on a generalization of the scattering approach which now includes the effects
of the (long-range) Coulomb interaction. We discuss the admittance of a wire
with an impurity and with a nearby gate. We extend a discussion of the
low-frequency admittance of a quantum point contact to investigate the effects
of the gates used to form the contact. We discuss the nonlinear I-V
characteristic of a resonant double barrier structure and discuss the
admittance for the double barrier for a large range of frequencies. Our
approach emphasizes the overall conservation of charge (gauge invariance) and
current conservation and the resulting sum rules for the admittance matrix and
nonlinear transport coefficients.
