Particle-in-cell (PIC) simulations in one spatial dimension of a cylindrical post-cathode direct current magnetron discharge have shown that there is a continuous transition from a positive space charge (PSC) mode of operation at higher pressures to a negative space charge (NSC) mode at lower pressures. In this paper we extend this work using a PIC code to investigate the NSC mode in detail. Profiles of ionization and inelastic collision rate, averaged charged particle energies and energy distributions indicate that the size and location of the region of power dissipation depend strongly on the operating pressure. Langmuir probe measurements of the radial distribution of floating potential in a laboratory magnetron discharge do not exhibit the systematic variation with changes in magnetic field or pressure that are predicted by the simulations. This implies the existence of an anomalous transport mechanism which maintains the PSC mode at low pressures and which cannot be modelled self-consistently by a PIC code in one spatial dimension.