  Linear conductance through a quantum dot is calculated under a finite
magnetic field using the modified perturbation theory. The method is based on
the second-order perturbation theory with respect to the Coulomb repulsion, but
the self-energy is modified to reproduce the correct atomic limit and to
fulfill the Friedel sum rule exactly. Although this method is applicable only
to zero temperature in a strict sense, it is approximately extended to finite
temperatures. It is found that the conductance near electron-hole symmetry is
suppressed by the application of the magnetic field at low temperatures.
Positive magnetoconductance is observed in the case of large electron-hole
asymmetry.
