true origin of the 0.25 and 0.85 conductance features which have been observed in biased split-gatequantum wires and quantum point contacts in semiconductor heterostructures is debated in the literature; onesuggestion is that they are caused by spontaneous spin polarization due to the electron-electron interactions. Thepresent work confirms that spontaneous spin splitting may occur within the system and is responsible for boththe 0.25 and 0.85 plateaux. We have also shown that the 0.25 plateau consists of two regions, one that is spinpolarized, and one that is degenerate with a conductance that remains essentially the same at both sides of thetransition. This finding could be of interest for semiconductor spintronics because it opens the possibility for spinmanipulation by electric means only.