  We study the electron spin relaxation in both symmetric and asymmetric
GaAs/AlGaAs quantum wells (QWs) grown on (110) substrates in an external
magnetic field B applied along the QW normal. The spin polarization is induced
by circularly polarized light and detected by time-resolved Kerr rotation
technique. In the asymmetric structure, where a {\delta}-doped layer on one
side of the QW produces the Rashba contribution to the conduction-band
spin-orbit splitting, the lifetime of electron spins aligned along the growth
axis exhibits an anomalous dependence on B in the range 0<B<0.5 T; this results
from the interplay between the Dresselhaus and Rashba effective fields which
are perpendicular to each other. For larger magnetic fields, the spin lifetime
increases, which is the consequence of the cyclotron motion of the electrons
and is also observed in (001)-grown quantum wells. The experimental results are
in agreement with the calculation of the spin lifetimes in (110)- grown
asymmetric quantum wells described by the point group Cs where the growth
direction is not the principal axis of the spin-relaxation-rate tensor.
