You have a couple of options:
Put a powered track, and right after that one a detector rail. That way, when your minecart will reach the powered (now unpowered) track, it will be slowed down as usual. But that doesn't happen instantly, and it will travel a little bit onto the powered rail, at which point it will trigger the detector rail, which will power the powered rail and you will continue.
This solution only works if you have exact control on the momentum and if you are coming fast enough.
Basically, this solution does slow down your minecart, IF your initial speed is higher than a certain limit. But the final speed will also depend on your initial speed, it does not always slow you down to a certain level, it rather slows you down a little bit.
Put a powered rail on a downslope, and before that (but not directly adjacent) put a detector rail. Then you'll have to setup a redstone delay circuit which will be activated by your detector rail, wait a second or so and then activate the powered rail, and since it is on a slope, you will start travelling in the direction of the slope.
You will have to experiment with the exact delay time (the powered rail must be activated after your cart has come to a complete stop, otherwise there will not be much slowing down), and also this solution does require the place and resources for the redstone circuit (not much though).
3. Make something I call a stopper track.
It works this way:
First if this was your initial track
you first make an extra track like this:
And wire the tracks on the intersection in such way that when you come from A, you go into B, but when you come from B you go into C. (This is easily done by some expreimenting, and if needed, single slopes, comment if u need more info on this).
Then all you have to do is that you put an upslope (with a powered track and a redstone torch if you wish) at the end of B.
This way, when you come from A, you go into B, and go to the upslope, but when you hit the end, you will loose all the momentum and speed. Directly after that you will start moving back into the intersection (cz you are in a slope), but this time with known speed (and not very high speed either)
Then it's just matter of putting the the powered rails at exact points to keep you cart moving forward at the lowest possible speed.
This solution requires the extra track, but it gives you what no other solution does: when you will travel out of the intersection you will ALWAYS have the same speed, no matter of what speed you had when you came in.
Just to clarify: The slope must end in a wall, like this:
otherwise your cart will not travel back and just stop there.
Screenshot of this: