  We experimentally investigate the dynamics of a persistent spin helix in
etched GaAs wire structures of 2 to 80 um width. Using magneto-optical Kerr
rotation with high spatial resolution, we determine the lifetime of the spin
helix. A few nanoseconds after locally injecting spin polarization into the
wire, the polarization is strongly enhanced as compared to the two-dimensional
case. This is mostly attributed to a transition to one-dimensional diffusion,
strongly suppressing diffusive dilution of spin polarization. The intrinsic
lifetime of the helical mode is only weakly increased, which indicates that the
channel confinement can only partially suppress the cubic Dresselhaus
spin-orbit interaction.
