  We have measured temperature ($T$)- and power-dependent electron spin
resonance in bulk single-wall carbon nanotubes to determine both the
spin-lattice and spin-spin relaxation times, $T_1$ and $T_2$. We observe that
$T_1^{-1}$ increases linearly with $T$ from 4 to 100 K, whereas $T_2^{-1}$ {\em
decreases} by over a factor of two when $T$ is increased from 3 to 300 K. We
interpret the $T_1^{-1} \propto T$ trend as spin-lattice relaxation via
interaction with conduction electrons (Korringa law) and the decreasing $T$
dependence of $T_2^{-1}$ as motional narrowing. By analyzing the latter, we
find the spin hopping frequency to be 285 GHz. Last, we show that the Dysonian
lineshape asymmetry follows a three-dimensional variable-range hopping behavior
from 3 to 20 K; from this scaling relation, we extract a localization length of
the hopping spins to be $\sim$100 nm.
