1. In a medium bowl, combine garlic, vinegar, sugar and 1/4 tsp. salt. Stir until sugar and salt dissolve. Add carrots and radishes, tossing to combine. Set aside to marinate.
2. Season chicken lightly with salt. Warm a nonstick grill pan over medium heat. Grill chicken until cooked through, 6 to 7 minutes per side. Transfer chicken to a cutting board, tent with foil and let rest.
3. Tear doughy centers out of bottoms of baguette. Lightly toast baguettes in toaster oven.
4. Thinly slice chicken. Spread 1 Tbsp. mayonnaise on 1 bottom piece of baguette. Top with 1/4 of cucumber slices and ¼ of chicken slices. Top with 1/4 of carrot mixture and a few cilantro leaves. Cover with top half of baguette. Repeat with remaining baguette pieces and filling.