  We study interaction-induced quantum correction to the conductivity tensor of
electrons in two dimensions for arbitrary T tau, where T is the temperature and
tau the transport mean free time. A general formula is derived, expressing the
correction in terms of classical propagators ("ballistic diffusons"). The
formalism is used to calculate the interaction contribution to the
magnetoresistance in a classically strong transverse field and smooth disorder
in the whole range of temperatures from the diffusive (T tau<<1) to the
ballistic (T tau>1) regime.
