  Using different techniques, and Fermi-liquid relationships, we calculate the
variation with applied magnetic field (up to second order) of the
zero-temperature equilibrium conductance through a quantum dot described by the
impurity Anderson model. We focus on the strong-coupling limit $U \gg \Delta$
where $U$ is the Coulomb repulsion and $\Delta$ is half the resonant-level
width, and consider several values of the dot level energy $E_d$, ranging from
the Kondo regime $\epsilon_F-E_d \gg \Delta$ to the intermediate-valence regime
$\epsilon_F-E_d \sim \Delta$, where $\epsilon_F$ is the Fermi energy. We have
mainly used density-matrix renormalization group (DMRG) and numerical
renormalization group (NRG) combined with renormalized perturbation theory
(RPT). Results for the dot occupancy and magnetic susceptibility from DMRG and
NRG+RPT are compared with the corresponding Bethe ansatz results for $U
\rightarrow \infty$, showing an excellent agreement once $E_d$ is renormalized
by a constant Haldane shift. For $U < 3 \Delta$ a simple perturbative approach
in $U$ agrees very well with the other methods. The conductance decreases with
applied magnetic field for dot occupancies $n_d \sim 1$ and increases for $n_d
\sim 0.5$ or $n_d \sim 1.5$ regardless of the value of $U$. We also relate the
energy scale for the magnetic-field dependence of the conductance with the
width of low energy peak in the spectral density of the dot.
