Ohmic contacts ballistic require no scattering as the charge carriers are transported through the channel, i.e. the length of the CNT should be much smaller than the mean free path (L<< l m). The opposite is valid for diffusive transport. In semiconducting CNTs at room temperature and for low energies, the mean free path is determined by the electron scattering from acoustic phonons, which results in l m ≈ 0.5μm. In order to satisfy the conditions for ballistic transport, one has to take care of the channel length and the properties of the contacts, while the geometry of the device could be any top-gated doped CNT FET.