Dependence of the mm flux density at 100 GHz on the source size and magnetic field strength for z = 0.016. The dashed horizontal line illustrates the region of source sizes ≥1 ld. The dashed vertical line delineates the range where the magnetic field ≤B_lim. The region between the refined values of R and B for the optically thick synchrotron regime and the observed flux of IC 4329A is highlighted in yellow.