The main assumption of the SN barrier approximation is that the electrostatic potential term takes the linear form Φ Φ = F x in the tunnelling region. The latter has been proved to hold only if x ≪ ≪ R . Therefore, if the tunnelling region has a length L , x < L for all x that determines the tunnelling process; thus if L ≪ ≪ R eq. (1) holds and the SN barrier approximation is valid. If the tunnelling probability is high enough to produce measurable field emission, L does not exceed 1-2 nm. Hence, the SN barrier is valid for emitters with radii of the order of some tens of nm.