By volume, there really isn't much difference between heavy cream and half and half when it comes to carbs per tablespoon, but you might find you can use a smaller volume of heavy cream. I use a tablespoon per cup (but it's an eating spoon, not a measuring spoon, that can differ, mine are only 2/3rds of a proper measured tablespoon).

I think there are sort of two concerns here, though. Atkins induction normally included restriction of cream, even heavy cream. But you can see that quite a bit of heavy cream could be included on a twenty grams or less of carbohydrate diet. Besides carbs, another problem with heavy cream is that the calories can add up fast. I once poured out a cup of heavy cream in the morning, to see how much I was using in my coffee through the day. I ran out before lunch time, 800+ calories--it's not all about the calories, but I do think that chaining caloric intake to something like caffeine can be a problem, you want to take in calories more because you're hungry, and less because that's how you happen to like your coffee. When I limited fat in my coffee, I was able to include more of it in my meals, making them more satisfying, and it helped me with weight loss.

One thing I do sometimes is to buy sugar free cashew or other nut "milk" and mix it half and half with heavy cream, that cuts carbs and calories considerably.