  Recently, we have introduced a novel inter-subband-induced spin-orbit (s-o)
coupling [Phys. Rev. Lett. 99, 076603 (2007); cond-mat/0607218] arising in
\textit{symmetric} wells with at least two subbands. This new s-o coupling
gives rise to an usual zitterbewegung -- i.e. the semiconductor analog to the
relativistic trembling motion of electrons -- with cycloidal motion without
magnetic fields. Here we complement these findings by explicitly deriving
expressions for the corresponding zitterbewegung in spin space.
