  We investigate the source of large variations in the observed detection
effiiencies of superconducting nanowire single-photon detectors between many
nominally identical devices. Through both electrical and optical measurements,
we infer that these variations arise from "constrictions:" highly localized
regions of the nanowires where the effective cross-sectional area for
superconducting current is reduced. These constrictions limit the DC bias
current density to well below its critical value over the remainder of the
wire, and thus prevent the detection efficiency from reaching the high values
that occur in these devices only when they are biased near the critical current
density.
