Origin of the Hall conductivity below the vortex-lattice melting in   twinned single crystal YBCO
  In his the Comment, P. Ao affirms that his vortex many-body theory explains the observations reported in G. D'Anna et al., Phys. Rev. Lett. 81, 2530 (1998). Indeed, we provide here some arguments to show the lack of a satisfactory explanation within the known theoretical frameworks. 
G. D'Anna, V. Berseth, L. Forro, A. Erb, E. Walker