  We analyze the four-fermion reactions e+e- -> 4f containing a single top
quark and three other fermions, a possible decay product of the resonant
anti-top quark, in the final state. This allows us to estimate the contribution
of the nonresonant Feynman graphs and effects related to the off mass shell
production and decay of the top quark. We test the sensitivity of the total
cross section at centre of mass energies in the t tbar threshold region and far
above it to the variation of the top quark width. We perform calculation in an
arbitrary linear gauge in the framework of the Standard Model and discuss an
important issue of gauge symmetry violation by the constant top quark width.

High Energy Physics - Phenomenology