  The time evolution of a local spin excitation in a (001)-confined
two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit
interactions of similar strength is investigated theoretically and compared
with experimental data. Specifically, the consequences of the finite spatial
extension of the initial spin polarization is studied for non-balanced Rashba
and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction.
We show that the initial out-of-plane spin polarization evolves into a helical
spin pattern with a wave number that gradually approaches the value $q_0$ of
the persistent spin helix mode. In addition to an exponential decay of the spin
polarization that is proportional to both the spin-orbit imbalance and the
cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the
spin polarization by a factor that approaches $\exp(-q_0^2 w^2/2)$ at longer
times.
