Group-quantization of non-linear sigma models: particle on S^2 revisited
  We present the quantum mechanics of "partial-trace" non-linear sigma models, on the grounds of a fully symmetry-based procedure. After the general scheme is sketched, the particular example of a particle on the two-sphere is explicitly developed. As a remarkable feature, no explicit constraint treatment is required nor ordering ambiguities do appear. Moreover, the energy spectrum is recovered without extra terms in the curvature of the sphere. 
V. Aldaya, M. Calixto, J. Guerrero and F. F. L\'opez-Ruiz