Open the cans of clams and drain the clam juice into a 2 cup measuring cup. Add enough water to make 2 cups of liquid. Set the clams and juice/water aside.
Turn on the pressure cooker's Sauté function and add the chopped bacon.
Cook, stirring occasionally, until fat has rendered out of it, but not crispy.
Add the butter, onion, celery, and thyme. Cook, stirring and scraping the bottom of the pot to get up all of the brown bits, until the onion starts turning translucent.
Add the garlic, salt, and pepper. Cook for 1 minute, stirring frequently.
Add the potatoes, sugar (if using) and clam juice/water mixture and stir.
Put the lid on the pot and lock in place. Turn the steam release knob to the Sealing position.
Cancel the Sauté function.
Set to Pressure Cook/Manual, and use the + or - (or dial) to choose 4 minutes (High Pressure). When cook time has ended, let the pot rest undisturbed for 2 to 3 minutes, then do a controlled Quick Release.
When the pin in the lid has dropped down, carefully open the lid, facing away from you, and stir the contents.
Use a potato masher to carefully mash some or all of the potatoes, as desired.
Turn the Sauté function back on, and use the Lowest setting (on the IP is called Less).
Add the clams and the half and half. Let the chowder heat through, but try not to boil it.
If you want it thicker, you can thicken it with potato or corn starch mixed with some of the hot chowder, mixed well, and stirred back in.
Turn off the pot and garnish the clam chowder with chopped chives, and serve with crackers or some nice bread or rolls.