▎ 摘　　要

We present the results of million-atom electronic quantum transport calculations for graphene nanoconstrictions with edges that are smooth apart from atomic-scale steps. We find conductances quantized in integer multiples of 2e(2)/h and a plateau at similar to 0.5 x 2e(2)/h as in recent experiments [N. Tombros et al., Nat. Phys. 7, 697 (2011)]. We demonstrate that, surprisingly, conductances quantized in integer multiples of 2e(2)/h occur even for strongly nonadiabatic electron backscattering at the stepped edges that lowers the conductance by one or more conductance quanta below the adiabatic value. We also show that conductance plateaus near 0.5 x 2e(2)/h can occur as a result of electron backscattering at stepped edges even in the absence of electron-electron interactions.