Nonlinear charging, and transport times in doped nanotubes junctions
  The nonlinear capacitance in doped nanotube junctions is calculated self consistently. It decreases as a function of the applied bias when the latter becomes larger than the pseudogap of the nanotube. For this device, one can deduce a relaxation time of about 0.1 femtosecond. Because of its negative differential resistance (NDR), a switching time of less than a fs can also be deduced. 
Keivan Esfarjani, Amir A. Farajian, Siu Tat Chui and Yoshiyuki Kawazoe