Jump same amount, no matter how long jump key is held

Hello, i would like to know how to make it so that, even in you hold down the space/ up key for any amount of time, the jump is always the same height, because at the moment if i were to hold down the space/up key for say 5 seconds, the object would never fall, and my game depends on the object jumping and falling the same height on every jump. Thanks

My Jump Code:

using UnityEngine;
using System.Collections;

public class Jumping : MonoBehaviour {

// Player Jump
public int jumpHeight;

// Update is called once per frame
void Update () {
	// Left Control
	if (Input.GetKey("up")){
		rigidbody.AddForce(new Vector3(0, jumpHeight,0), ForceMode.Force);
	}
}

}

You can use the isGrounded variable of the character controller, so the player can jump just after he fall on the ground.

   using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Update() {
        CharacterController controller = GetComponent<CharacterController>();
        if (controller.isGrounded)
            print("We are grounded");
        
    }
}