  We study theoretically the electron transport properties in carbon nanotubes
under the influence of an external electric field E(t) using Boltzmann's
equation. The current-density equation is derived. Negative differential
conductivity is predicted when {\omega}{\tau}<<1 (quasi-static case). We
observed this in the neighbourhood where the constant electric field E_o is
equal to the amplitude of the AC electric field E_1 and the peak decreases with
increasing E_1. This phenomenon can also be used for the generation of
terahertz radiation without electric current instability.
