GaAs nanowires were grown on GaAs (111)B substrates in a gas source molecular beam epitaxy system, using self-assembled Au particles with diameters between 25 and 200 nm as the catalytic agents. The growth rate and structure of the nanowires were investigated for substrate temperatures between 500 and $600^{circ}C$ to study the mass transport mechanisms that drive the growth of these crystals. The possibilities for fabrication of novel magnetic nanostructures by suitable choice of growth conditions are discussed.