  There currently is a large effort to explore spin-orbit effects in
semiconductor structures with the ultimate goal of manipulating electron spins
with gates. A search for materials with large spin-orbit coupling is therefore
important. We report results of a study of spin-orbit effects in a strained
InGaAs/InP quantum well. The spin-orbit relaxation time, determined from the
weak antilocalization effect, was found to depend non-monotonically on gate
voltage. The spin orbit scattering rate had a maximum value of $5\times
10^{10}s^{-1}$ at an electron density of $n=3\times 10^{15} m^{-2}$. The
scattering rate decreased from this for both increasing and decreasing
densities. The smallest measured value was approximately $10^9 s^{-1}$ at an
electron concentration of $n=6\times 10^{15} m^{-2}$. This behavior could not
be explained by either the Rashba nor the bulk Dresselhaus mechanisms but is
attributed to asymmetry or strain effects at dissimilar quantum well
interfaces.
