Thermal Conduction in a Tangled Magnetic Field

The suppression of thermal conduction by a static stochastic magnetic field is calculated for different ratios of the field scale length to the collisional mean free path. The effects of magnetic trapping are determined through a two-scale analysis and Monte Carlo particle simulations. In galaxy-cluster cooling flows, thermal conductivity is reduced from the Spitzer value by a factor of order ${10}^{2}$ to ${10}^{3}$.