  We experimentally demonstrate ultralong spin lifetimes of electrons in the
one-dimensional (1D) quantum limit of semiconductor nanowires. Optically
probing single wires of different diameters reveals an increase in the spin
relaxation time by orders of magnitude as the electrons become increasingly
confined until only a single 1D subband is populated. We find the observed spin
lifetimes of more than $200\,\textrm{ns}$ to result from the robustness of 1D
electrons against major spin relaxation mechanisms, highlighting the promising
potential of these wires for long-range transport of coherent spin information.
