  The quantum interference corrections to the conductivity are calculated for
an electron gas in asymmetric quantum wells in a magnetic field. The theory
takes into account two different types of the spin splitting of the conduction
band: the Dresselhaus terms, both linear and cubic in the wave vector, and the
Rashba term, linear in wave vector. It is shown that the contributions of these
terms into magnetoconductivity are not additive, as it was traditionally
assumed. While the contributions of all terms of the conduction band splitting
into the D'yakonov--Perel' spin relaxation rate are additive, in the
conductivity the two linear terms cancel each other, and, when they are equal,
in the absence of the cubic terms the conduction band spin splitting does not
show up in the magnetoconductivity at all. The theory agrees very well with
experimental results and enables one to determine experimentally parameters of
the spin-orbit splitting of the conduction band.
