Heat the oil in a large nonstick skillet over medium heat. Add the garlic and sauté 1 minute, just until fragrant.
Add the beef and sauté, breaking up gently, until no longer pink, about 5 minutes. Stir in the chili powder, salt, and pepper Add the root beer, tomato sauce, green chilies (if using), and Worcestershire sauce.
Bring to a lively simmer and cook about 15 minutes, stirring often, until thickened. Stir in the scallions. Pile onto rolls or buns. Serve with sweet-potato chips and prepared coleslaw.