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Electrical transport measurements were made on single-crystal Sn nanowires to understand the intrinsic dissipation mechanisms of a one-dimensional superconductor. While the resistance of wires of diameter larger than 70 nm drops precipitously to zero at T c near 3.7 K, a residual resistive tail extending down to low temperature is found for wires with diameters of 20 and 40 nm. As a function of temperature, the logarithm of the residual resistance appears as two linear sections, one within a few tenths of a degree below T c and the other extending down to at least 0.47 K, the minimum temperature of the measurements. The residual resistance is found to be ohmic at all temperatures below T c of Sn. These findings are suggestive of a thermally activated phase slip process near T c and quantum fluctuation-induced phase slip process in the low-temperature regime. When the excitation current exceeds a critical value …