JointDrive.positionSpring  

// Create a JointDrive, configure it and assign the JointDrive to
// the zDrive element of a configurable joint.

function Start() {
    var joint : ConfigurableJoint = gameObject.AddComponent(ConfigurableJoint);
    joint.targetPosition = Vector3(0,0,-10);
    
    var drive : JointDrive = JointDrive();
    drive.mode = JointDriveMode.Position;
    drive.positionSpring = 20;
    
    joint.zDrive = drive;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        ConfigurableJoint joint = gameObject.AddComponent<ConfigurableJoint>();
        joint.targetPosition = new Vector3(0, 0, -10);
        JointDrive drive = new JointDrive();
        drive.mode = JointDriveMode.Position;
        drive.positionSpring = 20;
        joint.zDrive = drive;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        joint as ConfigurableJoint = gameObject.AddComponent[of ConfigurableJoint]()
        joint.targetPosition = Vector3(0, 0, -10)
        drive as JointDrive = JointDrive()
        drive.mode = JointDriveMode.Position
        drive.positionSpring = 20
        joint.zDrive = drive