Peel half of the potatoes (leaving half unpeeled), and chop into 1-inch pieces. Add potatoes and chicken broth to slow cooker. Top with cubed butter and garlic.
Cover, and cook on high 3-4 hours, or until potatoes are tender
Using a potato masher or an electric mixer, mash potatoes to desired consistency. Stir in sour cream, salt, and pepper. Add enough milk until the potatoes are creamy. Serve warm.