Pour the wine and chicken stock into a large frying pan and bring it to a boil. When the mixture is just starting to boil, add the sugar and stir to dissolve. Add the chicken breasts to the mixture in the pan and simmer for 12-15 minutes or until the chicken is tender and the juices run clear when when a knife is inserted in the thickest part of the breast. Remove the chicken from the pan, set aside and keep warm.
Reheat the sauce and boil until reduced and thickened. Remove the string from the chicken and cut into slices (approx 3/4 to 1 inch thick).
Pour the sauce over the chicken and serve.