  We develop an approach for self-consistent ac quantum transport in the
presence of time-dependent potentials at non-transport terminals. We apply the
approach to calculate the high-frequency characteristics of a nanotube
transistor with the ac signal applied at the gate terminal. We show that the
self-consistent feedback between the ac charge and potential is essential to
properly capture the transport properties of the system. In the on-state, this
feedback leads to the excitation of plasmons, which appear as pronounced
divergent peaks in the dynamic conductance at terahertz frequencies. In the
off-state, these collective features vanish, and the conductance exhibits
smooth oscillations, a signature of single-particle excitations. The proposed
approach is general and will allow the study of the high-frequency
characteristics of many other low-dimensional nanoscale materials such as
nanowires and graphene-based systems, which are attractive for terahertz
devices, including those that exploit plasmonic excitations.
