  We have measured the resistance vs. temperature of more than 20
superconducting nanowires with nominal widths ranging from 10 to 22 nm and
lengths from 100 nm to 1050 nm. With decreasing cross-sectional areas, the
wires display increasingly broad resistive transitions. The data are in very
good agreement with a model that includes both thermally activated phase slips
close to Tc and quantum phase slips (QPS) at low temperatures, but disagree
with an earlier model based on a critical value of R_n/Rq. Our measurements
provide strong evidence for QPS in thin superconducting wires.
