  We use time dependent perturbation theory to study quantum size effects on
the terahertz nonlinear response of metallic graphene armchair nanoribbons of
finite length under an applied electric field. Our work shows that quantization
due to the finite length of the nanoribbon, the applied field profile, and the
broadening of the graphene spectrum all play a significant role in the
resulting nonlinear conductances. In certain cases, these effects can
significantly enhance the nonlinearity over that for infinitely-long metallic
armchair graphene nanoribbon.
