  We consider the effect of nucleation on a one-dimensional stepped surface,
finding that step-flow growth is metastable for any strength of the additional
step-edge barrier. The surface is made unstable by the formation of a critical
nucleus, whose lateral size is related to the destabilization process on a
high-symmetry surface. Arguments based on a critical nucleus of height two,
which suggest the existence of a fully stable regime for small barrier, fail to
describe this phenomenon.
