  We study spin relaxation and decoherence caused by electron-lattice and
spin-orbit interaction and predict striking effects induced by magnetic fields
$B$. For particular values of $B$, destructive interference occurs resulting in
ultralong spin relaxation times $T_1$ exceeding tens of seconds. For small
phonon frequencies $\omega$, we find a $1/\sqrt{\omega}$ spin-phonon noise
spectrum -- a novel dissipation channel for spins in quantum dots -- which can
reduce $T_1$ by many orders of magnitude. We show that nanotubes exhibit
zero-field level splitting caused by spin-orbit interaction. This enables an
all-electrical and phase-coherent control of spin.
